Shihao Su,Guojun Lv,Xuyang Zou,Jiangzhang Wang,Chaoyi Zhou,Yan Chen,Jialing Shen,Yangbin Shen,Zhongmin Liu
In the cyclic alkene epoxidation reactions using molecular oxygen as oxidant, aldehyde is generally added as co-oxidant to assist transition metal catalyst to activate the oxygen molecule, oxidizing cyclic alkene and producing the corresponding epoxide; however, the added aldehyde would be converted into byproduct carboxylic acid. Herein, we report the room-temperature tandem conversion of cyclic alkenes into 1,2-diols through the application of byproduct carboxylic acid as a catalyst for epoxide hydration reaction, realizing effective utilization of the byproduct and saving energy consumption. Four kinds of nanosized MnO2 specimens, including α-, β-, δ-, and γ-MnO2, were hydrothermally synthesized and applied as heterogeneous catalysts for the room-temperature tandem conversion of cyclohexene into 1,2-cyclohexanediol using molecular oxygen as oxidant and isobutyraldehyde as co-oxidant. The characterization results demonstrated that the obtained β-MnO2 sample had the maximum Mn4+/Mn3+ ratio of 1.42 and the minimum Oads/OLatt ratio of 0.34, consistent with its least oxygen vacancies evidenced by EPR measurement. Accordingly, the β-MnO2 sample revealed the quickest alkene epoxidation reaction rate and could achieve a 99.2% cyclohexene conversion within 1 h, and meanwhile, a 73.6% 1,2-cycloheanediol yield could be attained within 24 h. A plausible mechanism for the tandem conversion of cyclohexene into 1,2-cyclohexanediol over the β-MnO2 heterogeneous catalyst was proposed and further verified by quenching experiments and DFT calculations. Furthermore, the synthesized β-MnO2 catalyst sample could be reused more than ten times, and meanwhile, the tandem conversion way could be expanded to most of the cyclic alkene substrates, which demonstrated that the β-MnO2 catalyst was an active and stable heterogeneous catalyst for tandem conversion of cyclic alkenes into 1,2-diols.
Tandem catalysis reaction, which integrates two consecutive steps into a single one, can efficiently eliminate the separation of intermediate products, shortening the reaction time and decreasing energy expenditure. Therefore, the development of an efficient method for catalyzing tandem conversion of alkenes to prepare 1,2-diols is extremely fascinating. In this work, we reported the hydrothermal synthesis of tin-tungsten oxide catalysts at different pH values and their application as bifunctional catalyst for the one-pot tandem catalysis of alkenes to 1,2-diols. The SnWO 4 catalyst prepared at pH = 5 revealed the best tandem conversion performance and could attain a 95.4 % of cyclohexene conversion as well as a 77.2 % of 1,2-cyclohexanediol selectivity within 4 h of reaction. A possible mechanism corresponding to the tandem catalytic conversion of cyclohexene into 1,2-cyclohexanediol over SnWO 4 catalyst was proposed. Moreover, quenching experiments, density functional theory (DFT) calculations and in-situ infrared characterizations were further utilized to verify the reaction mechanism. Sn(II) could stabilize tungstate through the formation of W-O-Sn bonds and effectively reduced the energy of tungsten active centre binding to hydrogen peroxide, promoting the tandem catalysis reaction of cyclohexene to 1,2-cyclohexanediol. In addition, the one-pot tandem catalysis system could be further extended to the majority of linear and cyclic alkene substrates. This work can give a basis for the development of more green and efficient tandem conversion reaction of alkenes into 1,2-diols.
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